After setting this you want to update your system so the changes take effect:

root #emerge --ask --changed-use --deep @world

NoteIn the case of Radeon driver you additionally need to do the following

Radeon specific install steps

By default applications such as mplayer,vlc,vdpauinfo,... make VDPAU specific calls via libvdpau library.
This library then dynamically loads appropriate back-end driver ( VDPAU driver specific
to your hardware read more...).
At the time of writing the mechanism to automatically decide which back-end driver needs to be loaded
was not established. Currently libvdpau is hardcoded to load nvidia backend driver. It means that VDPAU will not work properly on Radeon cards.
The only possible way to change that behavior is by specifying the correct back-end driver manually.

First you need to find the name of the driver related to your hardware

(here we assume that the vdpau USE flag was enabled and the system was updated successfully).
One way to find that name is by checking Xorg log file:

user $cat /var/log/Xorg.0.log | grep -i vdpau

(II) RADEON(0): [DRI2] VDPAU driver: r300

The output will show if VDPAU driver has been initialized properly (two leading (II) letters in the sample output above). It also should specify the name of back-end driver (r300 in the sample output).

Now you can manually setup the name of back-end driver with help of VDPAU_DRIVER environment variable.

To do that you need to add the following line to ~/.bashrc file (provided that Bash is the default shell of a user who is going to run graphical environment). For the sample case described above the mentioned line would look like:

FILE~/.bashrcLoged as a user who run graphical interface

export VDPAU_DRIVER=r300

.

Now x11-misc/vdpauinfo should show you an information about your VDPAU configuration instead of an error message.